This invention relates to a gas turbine engine, and more particularly to the restoration of the dimensions of components of the gas turbine engine.
In a gas turbine engine, air is drawn into the forward end of the engine and compressed by a shaft-mounted axial flow compressor. The compressed air is mixed with fuel in the combustors, and the fuel is ignited. The resulting combustion gas flows through and turns a shaft-mounted axial flow turbine, which drives the compressor. The combustion gases flow from the aft end of the engine, driving it and the aircraft forward.
The turbine includes a turbine disk with turbine blades that project radially outwardly into the gas path of the combustion gas. An annular stationary shroud encircles the turbine blades and defines the gas path through which the combustion gas flows. The stationary shroud is circumferentially segmented. The stationary shroud segments are supported from the outer casing of the engine by a set of circumferentially segmented shroud hangers.
The shroud hanger segments are connected to the outer casing with an outer hook structure that allows these components to expand and contract at different rates without warping. Similarly, the shroud hanger segments and the stationary shroud segments are interconnected with an inner hook structure that allows these components to expand and contract at different rates without warping. These floating interconnections, rather than rigid welded or bolted interconnections, are required because of the radial temperature differentials experienced as the gas turbine engine is operated.
While this hook structure is operable and widely used, there are sometimes problems experienced because its required dimensions are not achieved in manufacturing or are lost during service. Similar dimensional-variation problems are experienced with other components of the gas turbine engine as well. There is accordingly a need for an improved approach to maintaining the dimensions of the shroud hanger segments and other structure in the engine. The present invention fulfills this need, and further provides related advantages.
The present invention provides a method for preparing a built-up gas turbine component in which a key dimension is brought within a specified dimensional tolerance. This approach produces a finished part whose built-up dimension is established to within close tolerances, without the need for final machining. The approach uses a non-line-of-sight technique. There is no chipping of the material, as may occur where thermal sprays are used. The process does not introduce any distortion in the finished built-up component. The approach may be applied to both nickel-base and cobalt-base alloys, and to a wide variety of types of components. Examples include shroud hangers, shrouds, and combustor components, with shroud hangers being of most interest.
A method for preparing a built-up gas turbine component includes providing a gas turbine component having a component surface and made of a component base metal having a component base metal composition. The gas turbine component may be either a newly made article or an article which has been in service and is being returned for repair and/or refurbishment. A buildup tape is supplied having a net metallic buildup composition different from the component base metal composition. The buildup tape includes a first metallic constituent having a first melting point, and a second metallic constituent having a second melting point. The first metallic constituent and second metallic constituent together comprise the net metallic buildup composition. The buildup tape additionally includes a nonmetallic binder binding together the first metallic constituent and the second metallic constituent. The method further includes applying the buildup tape to the component surface, and heating the buildup tape and the component surface to a brazing temperature greater than the first melting point and less than the second melting point. The first metallic constituent melts and fuses the first metallic constituent and the second metallic constituent to the component surface as a buildup deposit on the built-up gas turbine component.
The present approach is preferably practiced to adjust the dimensions of a shroud hanger having a forward hook structure including a forward radially outer hook structure having a forward outer hook land structure thereon, and a forward radially inner hook structure having a forward inner hook land structure thereon; and an aft hook structure including an aft radially outer hook structure having an aft outer hook land structure thereon, and an aft radially inner hook structure having an aft inner hook land structure thereon. The step of applying includes the step of applying the buildup tape to at least one of the land structures.
The gas turbine component may be made of a nickel-base superalloy base metal, and the buildup tape typically has a nickel-base alloy net metallic buildup composition. The gas turbine component may be made of a cobalt-base material, and the buildup tape typically has a nickel-base or a cobalt-base composition.
In one form, the nickel-base buildup tape has the first metallic constituent having a first-constituent composition, in weight percent, of from about 10 to about 30 percent chromium, from about 5 to about 12 percent silicon, balance nickel and minor amounts of other elements and impurities, and the second metallic constituent having a second-constituent composition, in weight percent, of about 99 percent by weight nickel, balance minor amounts of other elements and impurities. Preferably, the first metallic constituent has a first-constituent composition, in weight percent, of from about 18 to about 20 percent chromium, about 9.75 to about 10.5 percent silicon, balance nickel and minor amounts of other elements and impurities. The buildup deposit may be of any required thickness, but it preferably has a thickness of from about 0.001 inch to about 0.004 inch, and most preferably has a thickness of from about 0.002 inch to about 0.003 inch.
Thus, for example, a built-up gas turbine shroud hanger is made of a nickel-base superalloy base material and has a hook structure as described above. There is a shroud-hanger buildup deposit on at least one of the hook land structures. The shroud buildup deposit is made of a nickel-base alloy buildup material different in composition from the nickel-base superalloy base material, and is typically an alloy comprising nickel, chromium, and silicon. Other features of the invention as discussed above may be used with this embodiment.
The application of the shroud buildup deposit is most conveniently accomplished by furnishing a braze metal tape, and brazing the braze metal tape to the areas whose dimension is to be increased. The braze metal tape is a multi-component tape, such as a two-component tape, having a net composition required for the buildup material.
When shroud hangers are assembled into a gas turbine engine, it is crucial that the dimensions in the area of the land structures be precise, typically to tolerances of no more than +/xe2x88x920.001 inch. If the dimensions are outside of these tolerances, the shroud hanger typically does not fit together properly with the case and/or the shroud. New, as-cast and machined shroud hangers and shroud hangers that have seen service often have dimensions of the land structures that are outside of the tolerances in the areas of the land structures, and consequently do not function properly. If the dimensions of the land structures of the new shroud hangers are too large, the excess material may be machined away. If the dimensions of the land structures of new shroud hangers or shroud hangers that have returned from service are under the limits set by the dimensional tolerances, in the past it has been common practice to scrap the shroud hanger. The present approach provides a technique for repairing this problem and increasing the dimensions in the land structures of such shroud hangers, so that the dimensions are within tolerance and the article may be used in service.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.